There are innumerable little towns in California that cater to the tourist trade. Some of my favorites to visit have already been mentioned, like Healdsburg and Carmel.
Calistoga, CA has a population of around ~5500 and is the main town at the far northern end of the Napa Valley. It has numerous hot springs and some of my favorite wineries, including Chateau Montelena.
Mendocino, CA may have been mentioned already, as the exterior visuals setting for the T.V. show “Murder She Wrote.” It is on the coast, about ~3 hours from San Francisco. The Anderson Valley near it has some of California’s finest white wines and Pinot Noirs. Navarro is a must visit, if you like white wine. Anderson Valley Brewing Company is pretty tasty too. I don’t know if anyone still speaks Boontling in Boonville.
Marin and Sonoma Counties are lousy with little towns like Inverness, Tomales Bay, and Bolinas that are both indescribably pretty and have many things to do, such as kayak, dine on local oysters and cheeses, or bike ride in the bucolic hills. Bolinas is supposed to be rather unfriendly to tourists, but I didn’t have any issues when I drove through some 10 years ago.
Finally, the Eastern Sierra along highway 395, can look like a beer commercial. You know, the ones with impossibly high mountain peaks covered in snow, with rushing icy streams as cold as the beer they’re shilling? I like Bishop, CA as a less expensive way to explore many of the trout streams (though Bridgeport, CA is supposed to be better for that), trails, and sights like the Bristlecone Pine Forest,Devils Postpile, Mammoth Mountain, and Mono Lake. Plus, if you get bored with all that, drive over Tioga Pass into Yosemite. Should you go to Bishop, Mahogany Smoked Meats is well worth a stop, if you area jerky fan. Schaat’s Bakery is also a delicious place to stop, if you’re in the area.
If you’re an airplane buff,Mojave, CA has 3800 people, and a very large collection of aerospace companies/military bases. Edwards Air Force Base gives tours from time to time, information here.
Really though, you could write books on the touristy small towns in just one section of California. It just depends on what you’re interested in doing.